Optical sensors are used in many applications where control of position or detection of movement of an object is required. In some of these applications it is necessary to detect and control very small movements of objects with great accuracy. These applications require the use of very small optical sensors. Typically these very small optical sensors are semiconductor Charge-Coupled Devices (CCD's).
A typical CCD optical sensor is produced in an assembly that includes a support and a protective covering for the CCD, and a set of electrical leads that carry signals from the CCD. The combination of the support, the covering, and the leads is typically referred to as a package.
In the prior art, packages for CCD optical sensors were produced as either ceramic packages or molded packages. A ceramic package was produced by attaching leads to a ceramic disc, bonding the CCD to the ceramic disc and then placing a clear glass cover plate over the CCD to protect it. A molded package was produced by attaching a CCD to a set of leads and then molding a clear plastic housing around the CCD. Each of these prior art assembly techniques leaves something to be desired.
In the case of the molded package, there is a limit on the degree of refinement which is includable in the CCD optical sensor. In some medical and industry applications, a CCD optical sensor must include an integrated polymeric color filter array. These polymeric color filter arrays are not tolerant of the high temperatures encountered in a plastic molding operation. Consequently, a molded package is not a practical option for a CCD optical sensor that is adversely effected by high temperatures.
The prior art ceramic packages also have some shortcomings. A ceramic package must be assembled in a series of steps that requires a great deal of handling and manipulation of the package and its components which tends to increase cost and limit quality. Another problematic aspect of the prior art ceramic package is positioning the clear glass cover in the package. The glass cover is typically displaced at some distance from the CCD in a completed optical sensor. When this displacement distance is small, any imperfection or particle on the glass cover results in a false signal being produced by the optical sensor. As the displacement distance is made larger, there is a reduced likelihood that an anomaly on the glass will produce a false signal. In other words, when an anomaly is kept at a larger distance form the CCD, its deleterious effect is diffused. In prior art ceramic packages there is a practical limit as to the amount of displacement which can be introduced between a CCD and a glass cover on an optical sensor. This can be understood by considering an optical sensor 20 shown in FIG. 1.
Referring now to FIG. 1, there is shown a crosssectional view of a typical prior art optical sensor 20. The optical sensor 20 comprises a device 22, a support member 24, leads 25, and a glass cover 26. The support member 24 comprises a fired ceramic structure that is originally assembled from an unfired support plate 30 and one or more unfired annular rings 32. The support plate 30 and the annular rings 32 are shown as separate objects for purposes of clarity. In fact, the support plate 30 and the annular rings 32 become homogeneous when the support member 24 is fired.
For purposes of illustration, various dimensional references are made on FIG. 1. A letter S designates a distance between a top surface of the device 22 and a bottom surface of the glass cover 26. A letter H designates a height of one of the annular rings 32 and a letter W designates a width of the annular ring 32. It can be seen that the dimension S can be made increasingly large by making the support member 24 from a larger number of the annular rings 32 and by making each of the annular rings 32 with an increasingly large dimension H.
However, in the production of ceramic assemblies there are practical limits on the aspect ratios of various unfired parts that can be assembled into a final product. For example, an unfired annular ring made in accordance with conventional ceramic design rules would have its H and W dimensions approximately equal. This would assure a high yield of dimensionally correct product after firing. The design rule can be stretched to permit a dimension H which is up to 1.5 times the dimension W, but this results in a lower yield after firing and consequently a higher cost for the product.
Additionally, there are practical limits on how many of the annular rings 32 can be used to make one of the support members 24. When one of the annular rings 32 is placed onto one of the support plates 30, there is a certain probability that its position will be off-center. When a second one of the annular rings 32 is placed onto the first annular ring 32, there is a second probability that the second annular ring 32 is off-center relative to the support plate 30. This second probability is larger than the first probability because it is cumulative. Firing of the support plate 30 and the annular rings 32 produces additional dimensional variations that are cumulative with those introduced during assembly. There is a rapid reduction of yield of satisfactory support members 24 as the number of annular rings 32 used in the fabrication increases because of these cumulative probabilities for dimensional error. As a practical limit, it has been found that no more than two of the annular rings 32 can be used to produce one of the support members 24.
It is important to recognize how these ceramic design rules translate into design limitations for optical sensors in the context of small optical sensors, i.e., where the overall diameter or size of the support member 24 is less than about 0.25 inch. In this setting, it is desirable to utilize one of the devices 22 with as much surface area as possible. Thus, the annular rings 32 are made with the dimension W as small as possible, typically about 0.020 inch or smaller. This results in the dimension H of each of the annular rings being about 0.020 inch. A typical one of the devices 22 has a thickness of about 0.020 inch. Thus it can be seen that the dimension S is limited to 0.020 inch or less.
When the dimension S is only 0.020 inch or less, it is critically important to assure that the glass cover 26 is free of imperfections and that its surfaces are absolutely clean. Because of the very high standards needed to assure proper operation of the optical sensors 20, the glass covers 26 are inordinately expensive.
It is desirable therefore to produce an optical sensor with an increased tolerance for imperfections and surface anomalies in its cover. Additionally, it is desirable to produce such an optical sensor with more efficient and less costly manufacturing techniques.